The present invention relates to a knock sensing apparatus for sensing a knocking taking place in an internal combustion engine, suitable for use in ignition timing controller for optimizing the ignition timing. More particularly, with the knowledge that the pressure impulse or pulsation generated in the cylinder of the engine as a result of the knocking is transmitted to the cooling water through the cylinder wall and propagated through the cooling water, the invention aims at providing a knocking sensing apparatus taking place in an internal combustion engine through sensing the pulsation of the cooling water pressure.
As is well known, the internal pressure of the cylinder of the internal combustion engine is closely related to the ignition timing. In the explosion of the mixture in the cylinder after the ignition, the internal pressure of the cylinder does not include any higher harmonic provided that no knocking is taking place in the cylinder. The "higher harmonics" are the vibration components of frequencies normally ranging between 5 to 10 KHz and between 10 to 13 KHz, determined by the bore diameter of the cylinder and the sonic speed during the combustion. The higher harmonics are produced as a result of discontinuous quick combustion. If a knock takes place in the cylinder, the internal pressure of the cylinder comes to involve the higher harmonics when the internal pressure is increased to the level approximating the maximum combustion pressure. In consequence, a specific vibration or sound is transmitted to the outside of the cylinder. More specifically, in the initial state of the knocking referred to as "trace knock", the higher harmonics start to appear at the crank angle corresponding to the maximum combustion pressure. Then, as the knocking becomes stronger to the state called "light knock" and then to "heavy knock", the crank angle at which the higher harmonics appear is shifted to the advancing side, i.e. toward the crank angle corresponding to the ignition, from the crank angle of the maximum combustion pressure.
It is possible to remarkably improve the efficiency of the engine by optimumly controlling the ignition timing, through a precise detection of the shift of timing or the crank angle at which the higher harmonics appear.
However, as well known to those skilled in the art, there are some difficulties or problems in sensing the knocking through the detection of the pressure impulse, as explained hereinunder.
(1) It is true that the direct detection of the internal pressure of the cylinder by a pressure detector is the most efficient way of sensing the knocking. This, however, requires an independent detector for each of a multiplicity of cylinders of the engine, so that the cost of the knocking sensor is raised uneconomically. In addition, it is quite difficult to construct such a pressure detector of moderate cost as being capable of stably operating at high temperature and pressure for the controlling purpose. In fact, there still is no pressure detector commercially available and practically usable for the purpose described.
(2) It is also known that the knocking can be sensed indirectly through detecting, for example, the vibration transmitted to the outside of the cylinder. The engine block constituting the cylinder, however, has its peculiar vibration characteristics. Therefore, the change of the internal pressure caused by a knocking tends to be attenuated or filtered before it is picked up at the outside of the engine block. Namely, the sound/noise ratio S/N is lowered to make it difficult to sense the occurrence of knocking accurately. The signal picked up at the outside of the engine block, therefore, does not correctly represent the change of the internal pressure caused by the knocking, so that the ignition timing control system often fails to operate adequately. This problem is serious particularly when the engine has a multiplicity of cylinders. Namely, it is difficult to sense the knocking occurring in the cylinder remote from the position of the vibration detector. In addition, the noises such as valve down noise impedes the precise dection of the noise. Thus, this knocking sensing system is still unsatisfactory. Although it is possible to effect a feedback control of the ignition timing with this knocking sensing system, the control circuit tends to become complicated to avoid erroneous operation. Thus, for achieving the satisfactory knocking sensing performance, the system inevitably becomes more complicated and expensive.